1. Field of the Invention
The present invention relates to a process for the production of crosslinked polyurethane-polyurea powders by mixing certain NCO semiprepolymers with an aqueous medium, to the crosslinked polyurethane-polyurea powders prepared by this process and to their use as fillers in two-component polyurethane coating compositions.
2. Description of the Prior Art
Uncrosslinked polyurethane (PUR) powders are known. They may be produced, for example, by grinding elastomer granulates. The disadvantage is that the ground material has to be cooled with solid carbon dioxide or liquid nitrogen and the powders show unsatisfactory surface quality (cf. Kunststoffe 74, 195 (1984)). It is also known that aqueous polyurethane dispersions can be converted into PUR powders by spray drying. In this case, too, the powders become tacky under the effect of heat; their flow properties and their quality deteriorates accordingly (cf. for example Houben-Weyl, Methoden der organischen Chemie, 4th Edition, Vol. E20, page 1697 (1987)).
The production of uncrosslinked PUR powders in organic solution using special emulsifiers is disclosed, for example, in DE-OS 2 330 601 or U.S. Pat. No. 4,032,516. The "emulsifier/shear force process" is recommended in DE-OS 2 906 113 for the production of uncrosslinked PUR powders, while the production of uncrosslinked PUR powders by the acetone process is so described in DE-PS 2 226 526.
Previously, very little has been known concerning crosslinked PUR powders. According to DE-PS 1 282 962, solutions of slightly cationically modified NCO prepolymers in acetone are crosslinked with a mixture of diamines and triamines. The NCO prepolymers used have an NCO content of only 1 to 5% by weight, so that only slightly crosslinked polyurethane polyureas having softening or melting ranges of about 90.degree. to 180.degree. C. are obtained as the end products of the process.
It has now surprisingly been found that it is possible to obtain valuable crosslinked polyurethane-polyurea powders by mixing certain isocyanate semiprepolymers, which are locally nonionically/hydrophilically modified, with water. The resulting polyurethane-polyurea powders are valuable fillers for coating compositions of any kind, especially for two-component polyurethane coating compositions, and enable coatings having particularly good surface properties to be obtained.